While the invention is subject to a wide range of applications, it is especially suited for use in amplitude modulation broadcasting systems where it is desirable to provide large amounts of envelope modulation and will be particularly described in this connection.
Broadcasters are limited, by their power and frequency assignments, in their attempts to cover as large an area as possible. Accordingly, broadcasters in general, and AM broadcasters specifically, have tended to use sophisticated audio processors for maximizing effective modulation.
The present rules covering AM medium wave broadcasting in the United States allow broadcasters to produce up to 100% negative going modulation and 125% positive going modulation.
Audio processing equipment, according to one preferred embodiment of this invention, will alter the waveshape of the input audio signal in such a manner as to cause high effective modulation with positive going peak modulation approximately 125% and peak negative going modulation approximately 100%, the limit allowed by many Governmental regulatory bodies.
Presently, there are two basic procedures used for producing asymmetrical modulation. The first is to use two clippers, one set to clip at a voltage level providing 125% positive going modulation, and the second at minus 100% modulation. The audio level is then set high enough to insure that the audio input reaches and exceeds the clipping level of the 125% clipper. Thus, if the signal into the clippers is essentially symmetrical, the clippers would, by their asymmetrical adjustments, create an asymmetrical modulated wave.
The second procedure is to use the essential asymmetrical nature of certain audio signals, such as, many speech waves produced by good radio voices. It is, however, necessary to insure the proper polarity of the asymmetrical characteristic to provide higher positive modulation. Equipment has been in use, for a number of years, which automatically switches the phase of the audio wave by 180 degrees, whenever the polarity of the wave is wrong.
These procedures avoid the use of clippers but do not insure asymmetrical modulation for all signals nor do they insure the correct amount of asymmetry. AM broadcasters often use a combination of the first and second basic procedures but this introduces clipper distortion products.